RDP from xp to vista with authentification on network level

Hello, my English is bad but I hope I can explain my problem.

I have new laptops with Vista. I would like configure the RDP with (German:
Authentifizierung auf Netzwerkebene) authentification on network level. If I
take a Vista Client everything is o.k. If I take an XP Client I get a Window
how tells, that XP can't this. I do not find an RDP for XP with this
function.

Where can I find an RDP-Client for XP?

I need this function. We often works with RDP.
The version (6.0.6000.1638) of my mstsc.exe is the same on XP and Vista.

Thanks for help.
Gisela